Help with retrofit composite chainplate design

[Jagarwal]

Hello.

Strip planked 47ft wood/fibre cat. Big double spreader monohull style sloop rig. Wire was 10mm stainless, forestay was 12mm.

My chainplates are original to the boat and since the mast is down Ive decided Id like to replace them. I was wondering if it would be possible to replace them with a composite alternative. The chainplates are located in pairs in line with the mast and have 4 shrouds terminating at each pair

Currently the chainplates are tied into my main crossbeam which is essentially a timber plywood box beam with a series of trusses (think bridge) running between the webs. I have attached a diagram to illustrate the setup. The current setup basically wraps around the beam.

My proposed replacement is to cut slots in the bridgedeck and deck on either side of the beam, then loop around the beam using unidirectional S glass (Or carbon if I can be convinced that its worth it), passing under the bottom of the bridgedeck and over the deck + some sort of mandrel setup for rigging pins. Another drawing attached. In my mind this seems to be a pretty simple replacement of the function of the stainless steel. Since its one layup and it wraps all the way around it should pull up on the whole beam just like the stainless does.

I don't have much experience with boat design and want to make sure I am not on a bad path here.
I prefer going composite over stainless as its cheaper, lower future maintenance, and hopefully truly leak free. I do have fiberglassing experience and work with epoxy quite a lot. It will certainly be a multiperson job.

Can someone point me to a resource to help understanding strength calcs and working towards a layup schedule?

I am unsure about turnbuckle attachment, currently my turnbuckles take a 15mm pin, will that diameter be big enough to build the chainplate around?

What am I not considering or missing?

Thanks in advance

 

[montero]

Hi ! Jagarwal .There's concave in wraps over up between pins . It' s difficult to laminate and potentially weak point .
In existing setup , tension rods they pierce lower bridgedeck . Going continious fibers that way will be difficult and probably not necessary.

 

[Ilan Voyager]

Why going into complications, while the hardware is so easily made in stainless steel for an affordable price?

 

[Jagarwal]

Why going into complications, while the hardware is so easily made in stainless steel for an affordable price?

I am in Panama, the hardware is not easily made here. And while the price is not outrageous, it will probably be around $4,000. I think the composite solution would come in at under 500 and doesnt require running around trying to source materials.

In execution the composite solution is no more complicated than that of the stainless, its just not very familiar and not the typical way.

 

[montero]

Wraping around will be difficult to make using continuous fiber. You can use U shaped pieces , one layer from above , next from the bottom . Multi layer will achieve very high strenght .

 

[Ilan Voyager]

I saw the drawings. It's very complicated you'll suffer to make that.
First the slots will leak water soon or later.
Second I let you the pleasure to struggle with the strands, and most important to get an high quality composite on a piece submitted to substantial stresses.
Third the $500 is largely underestimated. You need UD carbon fiber in quantity, and some 45° glass. Plus a high strength epoxy resin, which are generally "cooked" cured. The final composite needs to be microscopic air bubble free, but in your case vacuum is almost impossible, so the composite will be rather in the side of low quality. So you'll need more material and to take big security margins, at least 3.
The main problem with composites is the final quality. With the same material you can get a result from piece of junk to state of the art. And for getting high quality that means to be extremely careful in a very clean environment with controlled temperature and humidity. I know the problem as I have worked with composites since 1980 as I've worked in the field of racing oceanic multihulls.
It will be far easier to make the job in stainless steel 316L. Nowadays with the good cutting disks 1 mm thick for SS, the cutting operation needs just a 125 mm angle grinder and some patience. SS316 is one of the easiest metals to weld perfectly with 316L rods and a cheap 140 amps 200 dollars inverter MMA welder. With the tungsten carbide bits making the holes is now a breeze. That can be done in any ordinary shop or by yourself except maybe the welds, but I'm sure that you can find a professional welder in Panama. All this tooling can be bought at good price in AliExpress.
And the most important SS316 is a isotropic material, so you have not to take care of fibers, interlaminations peeling, microscopic air bubbles. Any direction in SS is as strong as the other, the weld is as strong as the base material if done with the easy due precautions. And it's an industrial product with predictable properties so the engineering calculations are simple.

 

[AndrewK]

Jagarwal, If you are really convinced that you want to replace what is already there with composite then I don't see it being that difficult to what you want to do.

In the past 20 years in Australia most home built catamarans go with composite, a lot are low tech E glass no vacuum consolidation.

There should be lots of information here on Boatdesign.
A link to a low tech approach is here, "The Scrumble Project https://scrumbleproject.wordpress.com/

If your wire is 10mm then a laminate that is 5 times stronger is a good starting point, some qualified members may come along and offer help with the calculations.

Cheers
Andrew

 

[montero]

@Jagarwal Please provide more photos. Thinking that fiberglass fabric is stronger than steel doesn't make much sense in this case. How is the crossbeam constructed?

 

[Milehog]

There is a fella with a YouTube channel that built composite chainplates. The design and build span a number of episodes.
Maybe you can get some ideas there.
Start looking here:

 

[Jagarwal]

Hello gentlemen and thank you for the excellent resources.

Montero, the crossbeam is essentially a box beam, laminated timber top and bottom with plywood webs and a warren truss running between the upper and lower members. I hope my drawing is clear, I am not sure how to give more detail.

I am on the optimistic side of the argument and think that its not unachievable.
I find Matt's (Duracell Project) approach straightforward. The surfaces to which the chainplates would mount are vertical so I could make the chainplates outside the boat as he did and then install them. I would use his 5x strength of the wire to calculate my layup thickness. Any resources related to strength calcs for these kinds of layups is appreciated.

One of the issues I face is that the current chainplates are bolted through the beam. I assume this is how much of the load is transferred to the beam. Since the webs of the beam are plywood, there is no way to laminate the chainplates in without relying only on the strength of the outermost ply. Is it possible to build the composite replacement in a way that it can be through bolted as the stainless was? How is this typically achieved or is it just a no go with composites?

I understand the posters noting that there are simpler, easier solutions. I like to explore solutions that also create opportunities to learn. Once I can fully understand the pros and cons for both approaches I will decide what solution I want to proceed with.

 

[AndrewK]

Jagarwal, my first thought was to make a short composite to go through the deck and connect to the existing tie rod, but then I changed my mind. Reason being that this still involves modifying SS that you want to avoid.

Basically you are making a long "U" bold out of composite.
You can do this two ways;
1. make a mock cross section of the beam and use this as a former to make the composite chain plate. Then glue into place.
2. fix the 2 SS bushing tubes firmly at a separation the total length the "U" is to be in a vertical jig. Then wind the UD and +/-45 tapes firmly around the two to form a long strap.
Pass each end through the bottom and pull up through the deck. At deck level have a tensioning jig to pull it up tight against the beam while it cures.

I can make a drawing and add more details if you want. Also forget the S glass as it costs more than carbon these days, and carbon is also hard to justify on a cruising boat as you only save 30% of the weight.

Andrew

 

[Jagarwal]

Jagarwal, my first thought was to make a short composite to go through the deck and connect to the existing tie rod, but then I changed my mind. Reason being that this still involves modifying SS that you want to avoid.

Basically you are making a long "U" bold out of composite.
You can do this two ways;
1. make a mock cross section of the beam and use this as a former to make the composite chain plate. Then glue into place.
2. fix the 2 SS bushing tubes firmly at a separation the total length the "U" is to be in a vertical jig. Then wind the UD and +/-45 tapes firmly around the two to form a long strap.
Pass each end through the bottom and pull up through the deck. At deck level have a tensioning jig to pull it up tight against the beam while it cures.

I can make a drawing and add more details if you want. Also forget the S glass as it costs more than carbon these days, and carbon is also hard to justify on a cruising boat as you only save 30% of the weight.

Andrew

Hey Andrew

I believe I understand, Essentially make a giant U outside the boat and then pass it up from below. This makes sense to me, and can essentially lay it up like matt did and then lay it over the form. Drawing is maybe helpful for extra details. Each chainplate location supports 4 shrouds; Should I make 1 "U" per pair of shrouds (4 total) or its okay to put 4 on a single U of wider dimension? What are the pros and cons here?

Any feedback on if bolting through the chainplates will be possible?

thanks
Jack

 

[AndrewK]

Jack, I would make a single "U" of wider dimension, but 3 say 50mm wide spaced the width of the turnbuckle would work.
What is the depth and width of the beam, and also the width of the current unpainted chainplate area?

"Vectorlam" is a free software from Vectorply, worth having a look at.

 

[AndrewK]

What is the angle of the lower shrouds? this will need consideration as composite chainplates are normally directly in line with the shrouds.